Degradation of picosecond temporal contrast of Ti:sapphire lasers with coherent pedestals

Delay-shift and asymmetric broadening of pre-pulses by post-pulses in a petawatt laser facility

The temporal contrast of high-peak-power lasers is usually limited by pre-pulses, which are generally produced by post-pulses due to the nonlinearity of the active medium. The reason for the conversion between pre-pulse and post-pulse is now well known, but the mechanisms for the delay-shift and asymmetric broadening of the newly generated pre-pulse are not yet clear. In this work, a novel, to the best of our knowledge, numerical model combining the nonlinear Schrödinger equation and the Frantz-Nodvik equation is proposed to investigate the underlying mechanisms for the "distortion" of the pre-pulse. Numerical results show that the gain characteristics of Ti:sapphire amplifiers can only make a minor change on the temporal profile of the pre-pulse, but the high-order dispersion is the main cause for the delay-shift and asymmetric broadening of the pre-pulse, and the effects are more significant for the initial post-pulse with a relatively larger delay.

Reducing temporal pedestal in a Ti:sapphire chirped-pulse amplification system by using a stretcher based on two concave mirrors

We demonstrate the temporal pedestal suppression in a Ti:sapphire chirped-pulse amplification laser system. The far-field spectral phase noise can be avoided by using a stretcher based on two concave mirrors in the system, reducing the intensity and the time range of the amplified pulse's pedestal. At the amplified energy of 1.7 J, the contrast measurement showed that the pedestal intensity was at a level of about 10^-10 within a 10 ps time window near the main pulse. In the proton acceleration experiment, a 10 nm thickness CH target was irradiated by the high-contrast pulse with the focused intensity of ∼1.4×10²⁰W/cm², which generated a proton beam with a cutoff energy of 16 MeV.

Temporal prepulse contrast degradation in high-intensity CPA lasers from anisotropy of amplifier gain media

We present a study of the temporal prepulse contrast degradation of high focused intensity pulses produced in CPA laser systems due to imperfections in amplifier design, alignment of amplifier components, and crystal inhomogeneity. Using an extended cross-polarized imaging technique, we demonstrate the presence of multiple crystal domains inside Ti:sapphire slabs with ≈10cm diameter. The results of our numerical calculations show that crystalline c-axis orientation inhomogeneity caused by these crystal domains can lead to the generation of prepulses with a relative contrast of >10^-10 within several picoseconds before the main pulse. In a multiple-slab amplifier head configuration sometimes used in high-repetition-rate systems, the misalignment of the crystalline c-axes of the amplifier slab with respect to each other can lead to the generation of prepulses with relative contrast as high as 10^-6, depending on the magnitude of misalignment.

Petawatt Femtosecond Laser Pulses from Titanium-Doped Sapphire Crystal

Ultra-high intensity femtosecond lasers have now become excellent scientific tools for the study of extreme material states in small-scale laboratory settings. The invention of chirped-pulse amplification (CPA) combined with titanium-doped sapphire (Ti:sapphire) crystals have enabled realization of such lasers. The pursuit of ultra-high intensity science and applications is driving worldwide development of new capabilities. A petawatt (PW = 1015 W), femtosecond (fs = 10−15 s), repetitive (0.1 Hz), high beam quality J-KAREN-P (Japan Kansai Advanced Relativistic ENgineering Petawatt) Ti:sapphire CPA laser has been recently constructed and used for accelerating charged particles (ions and electrons) and generating coherent and incoherent ultra-short-pulse, high-energy photon (X-ray) radiation. Ultra-high intensities of 1022 W/cm2 with high temporal contrast of 10−12 and a minimal number of pre-pulses on target has been demonstrated with the J-KAREN-P laser. Here, worldwide ultra-high intensity laser development is summarized, the output performance and spatiotemporal quality improvement of the J-KAREN-P laser are described, and some experimental results are briefly introduced.

Improvement in the temporal contrast in the tens of ps range of the multi-PW Apollon laser front-end

We demonstrate the impact of the optics roughness in Öffner stretchers used in chirped pulse amplification laser chains and how it is possible to improve the temporal contrast ratio in the temporal range of 10-100 ps by adequately choosing the optical quality of the key components. Experimental demonstration has been realized in the front-end source of the multi-petawatt (PW) laser facility Apollon, resulting in an enhancement of the contrast ratio by two to three orders of magnitude.

Experimental investigation on the temporal contrast of pre-pulses by post-pulses in a petawatt laser facility

We experimentally explore the generation of pre-pulses by post-pulses, created through internal reflection in the optical components, by the nonlinear process associated with the B-integral in the laser chain of the petawatt (PW) facility J-KAREN-P. At a large time delay between the main and the post-pulses, we have found that the pre-pulses are not generated from their counterpart post-pulses at an identical time difference before the main pulse, and the temporal shapes of the pre-pulses are greatly distorted asymmetrically. We have also observed that the peak intensities of the pre-pulses are drastically suppressed compared to the expected value at a small time delay. We briefly describe the origins of the pre-pulses generated by the post-pulses and demonstrate the removal of the pre-pulses by switching to optical components with a small wedge angle at our PW laser facility.

Modeling and Analysis of High-Power Ti:sapphire Laser Amplifiers–A Review

We have introduced several factors that can be useful for the modeling and analysis of high-power Ti:sapphire laser amplifiers. The amplification model includes the phase distortion effect caused by the atomic phase shift (APS) in gain medium and the thermal-induced phase distortion effect caused by the high-average-power amplification. We have provided an accurate amplification model for the development of ultra-high-intensity and high-average-power lasers.

Investigation of the temporal contrast evolution in a 10-PW-level Ti:sapphire laser facility

We have theoretically and experimentally investigated the evolution of the temporal contrast in a 10-PW-level Ti:sapphire laser in the Shanghai Superintense Ultrafast Laser Facility (SULF). The effects induced by the grism pair, spectral shaping filter, and increase in gain on the temporal contrast were investigated. First, it was found that the energy loss of clean seed pulses in the grism pair is a major factor in contrast degradation. Because of the low transmission efficiency of the grism pair (~10%), the temporal contrast is degraded by one order of magnitude. Second, the spectral shaping filter in the regenerative amplifier degrades the temporal contrast by increasing the intracavity loss. Finally, as the amplified spontaneous emission pedestal experiences gain more than the main pulse in Ti:sapphire amplifiers, particularly during saturated amplification, the temporal contrast will further deteriorate as the gain increases in multi-stage Ti:sapphire amplifiers. In addition, the effect on the temporal contrast induced by the extraction during pumping technique in large-aperture Ti:sapphire amplifiers has been considered. According to the investigations described above, the design of the SULF can be further improved. It is predicted that a temporal contrast of over 10^-11 can be achieved at a peak power of 10 PW following the improvements. The investigations conducted in this study can provide guidelines for improving the temporal contrast in ultrahigh-peak-power Ti:sapphire lasers.

Spatiotemporal coherent noise in frequency-domain optical parametric amplification

Frequency-domain optical parametric amplification (FOPA) is a new scheme that enables extremely broadband amplification of ultraintense pulses. The spatiotemporal coupling property of signal pulses can make the coherent noise of FOPA sharply different from that of conventional OPCPA. This paper presents a first theoretical study on the coherent noise produced in a FOPA system. We reveal that the coherent noise acquires the spatiotemporal coupling, and thus distinguishes the compressed signal pulse not only in time but also in space, which allows the suppression of coherent noise via optical manipulations in the spatial domain. The quantitative impacts of spatiotemporal coherent noise originated from the imperfections in either pump laser or crystal surfaces, are numerically studied. The result provides a new perspective on improving the coherent contrast of ultraintense lasers.

Nonlinear beat noise in optical parametric chirped-pulse amplification

The pulse contrast of state-of-the-art petawatt lasers is limited by coherent noise. This paper reports on a new family of noise, termed nonlinear beat noise, which is generated by the nonlinear mixing of two kinds of coherent noise in optical parametric chirped-pulse amplification (OPCPA). We theoretically study the various nonlinear beat noises and reveal their intensity evolutions in an OPCPA amplifier. The results suggest that nonlinear beat noise will be destructive to the future hundred-petawatt lasers.

Reflection of few cycle laser pulses from an inhomogeneous overdense plasma

The use of a plasma mirror to improve the temporal contrast of few cycle laser pulses has been considered. Pre-plasma features, prior to the main pulse, have been evaluated using an analytical model that has been verified using hydrodynamic code. The temporal/ spectral profile, reflectivity, and broadening of the reflected pulse have been parametrically analysed using an analytical formulation that describes the reflection of broadband ultra-short pulses from the plasma gradient. The analytical estimate for the pulse reflectivity is in good agreement with experimental measurements. The consistency of the analytical expressions for the collisionless case has been validated via comparison with a 1D particle in cell simulations.